Multiplex systems of both the time division and frequency division types are well known in the art in connection with full duplex communication systems. Asynchronous multiplex systems, such as that disclosed in U.S. Pat. No. 4,097,695, are also known in which the bits within each character or block of characters relates to a fixed time frame, but wherein the start of each character or block thereof is not necessarily related to such fixed time frame.
Typically, asynchronous multiplex systems avoid the need for transmitting high frequency clock signals between terminals be employing integrated circuits know as universal, asynchronous receiver transmitters (UART's). These UART circuit subsystems have simultaneous receiving and transmitting capabilities, and provide for converting data words serially received into parallel format, and for serially transmitting data words input to the UART in parallel format. Although timing clocks are necessary, the clocks employed by each UART in a pair of data terminals between which information is communicated need not be synchronous; thus, independent clock generators can be used and no clock transmission is required.
In connection with prior art, asynchronous multiplex systems, it was often necessary to include a plurality of address bits as part of the data message output from a UART of a terminal transmitting data in order to access the proper terminal at the other end of a data link which is to receive such message.
Accordingly, it is a primary object of the present invention to provide a multiplex system of the asynchronous type in which any of a group of data terminals may efficiently communicate with a master station or a data terminal in another group thereof.
Another object of the present invention is to provide a multiplex system of the type described above in which data is transmitted from any of a group of data terminals in accordance with the sequence of the formation of data words at such terminals.
A still further object of the present invention is to provide a system of the type mentioned which employs a plurality of universal, asynchronous receiver-transmitters respectively associated with each data terminal in a group thereof for serially receiving and temporarily storing data bits from the corresponding data terminal.
A further object of the present invention is to provide a multiplex system as described above which includes a scanner for successively scanning the UART's to detect the presence of a data word stored therein which is ready for transmission, such that data words are transmitted in an asynchronous manner determined by the sequence in which such data words are formed.